1. Field of the Invention
The present invention relates to a digital/analog converter, and more particularly, to an oversampling digital/analog converter using an Interpolated Finite Impulse Response (IFIR) filter.
2. Background of the Related Art
A related art oversampling digital/analog converter will now be described. FIG. 1 illustrates a system block diagram of the related art digital/analog converter (DAC). A sigma delta DAC is generally used as a digital/analog converter.
Referring to FIG. 1, related art the sigma delta digital/analog converter includes an interpolation filter 11, a digital noise shaper 12 and an FIR reconstruction filter 13. The interpolation filter 11 receives a multibit digital word of first sampling frequency and converts it to a multibit digital word of a second sampling frequency higher than the first sampling frequency. The second sampling frequency multibit digital word is then converted into a single-bit word in the digital noise shaper 12. The single-bit quantization used for the conversion in the digital noise shaper 12 shifts a quantization noise from a low frequency band to a high frequency band (noise shaping). The FIR reconstruction filter 13 has low pass filters, which either have a switch-capacitor or have a resistor and a capacitor. The low pass filter with the switch-capacitor, which has a non-linear phase response, is embodied as a CMOS integrated circuit. The low pass filter with the resistor and the capacitor provides a wider dynamic range, but it requires a precise matching between components for precise filtering.
FIG. 2 illustrates a related art Finite Impulse Response (FIR) type reconstruction filter. Referring to FIG. 2, the related art FIR type reconstruction filter includes a plurality of one bit shift registers SR1, SR2, SR3, . . . , SRn connected in series, a plurality of current sources CS1, CS2, CS3, . . . , CSn that each supply a respective current to an I-V converter part 21 or drain the respective current to ground in response to a signal from the shift register SR1, SR2, SR3, . . . , SRn. The I-V converter part 21 converts the selectively received current depending on respective outputs of the shift registers SR1, SR2, SR3, . . . , SRn to a voltage.
The operation of the related art oversampling digital/analog converter will now be described. First, the related art oversampling digital/analog converter subjects a one bit data stream to low-pass filtering using a FIR semi-digital reconstruction filter and subjects a resulting current to I-V conversion. That is, as shown in FIGS. 1 and 2, a noise shaped digital data provided to the FIR reconstruction filter 13 is converted into an analog signal. In other words, the noise shaped digital data stream is provided to the shift registers SR1, SR2, . . . , SRn in the FIR reconstruction filter 13. If the shift register provides a data "0", the current from a current source of the shift register is connected to ground. For example, if the first shift register SR1 provides a data "1", the second shift register SR2 provides a data "0" and the third shift register SR3 provides a data "1", paths of the first and third current sources CS1 and CS3 are established toward the I-V converter part 21 and a path of the second current source CS2 is established to the ground. Therefore, the currents from the first current source CS1 and the third current source CS3 are together provided to the I-V converter part 21. The I-V converter part 21 converts the received current into a voltage corresponding to the digital data stream received at the shift registers. In this instance, to convert the digital data into a voltage corresponding to the digital data with more precision, more current sources are required. That is, the more orders the filter is extended, the more exact the analog output can be obtained.
In summary, as shown in FIG. 1, digital data is provided to the FIR reconstruction filter 13 through the interpolation filter 11 and the digital noise shaper 12. The FIR reconstruction filter 13 subjects the digital data to low pass filtering according to a transmission function. The transmission function of the FIR reconstruction filter can be expressed as equation 1 as follows. EQU H(Z)=a.sub.1 z.sup.-1 +a.sub.2 z.sup.-2 +, . . . , a.sub.n z.sup.-n (1)
Therefore, when the noise shaped digital data is passed through the FIR reconstruction filter 13, a high frequency component in the noise shaped digital data is removed, which leaves a baseband signal.
However, as described above the related art oversampling digital/analog converter has various problems. First, the high order of FIR reconstruction filter required for conversion of a signal from digital to analog results in an increase of occupied area due to the filter system. Second, the error in a filter coefficient caused by process change coming from increased order degrades a filter performance. Third, the current to the I-V converter part being at least greater than "0" at the minimum and smaller than a sum of all current sources at the maximum places a limitation on a dynamic range of the converted voltage signal.